Like anti-lock braking systems (ABS) before them, traction control systems are becoming increasingly common in vehicles such as trucks, vans, and automobiles. In ABS, wheel lock-up caused by over-application of the brake pedal or the encountering of a road surface with less than optimal traction during braking is sensed by the ABS sensing elements. The circuitry associated with these sensing elements closes the normally open isolation valve located between the master cylinder and brake slave cylinder and at the same time, activates the high pressure ABS pump and opens the normally closed hold/dump valves. To release the brakes, the high pressure in the brake slave cylinder initially dumps into the low pressure accumulator. However, if the accumulator becomes filled with fluid, the slave cylinder fluid will no longer have an escape path. Therefore, the high pressure pump pumps fluid from the accumulator back to the master cylinder reservoir. The brakes thus momentarily release, restoring wheel rotation. In a closed ABS system, the hold/dump valve is then closed and high pressure fluid from the pump reengages the brakes. This cycle repeats itself as long as brake pedal pressure is applied and wheel lock or incipient wheel lock is detected.
In a traction control system (TCS), the sensing elements activate traction control (TC) when wheel spin is sensed. The TCS momentarily applies pressurized brake fluid to the brake slave cylinders, slowing wheel rotation, and restoring traction. Like ABS, the brakes may rapidly cycle between brake apply and release cycles, however in TCS, the cycling continues until the sensing elements no longer detect wheel spin rather than wheel lock. Unlike ABS, however, where a pressurized supply of brake fluid from the master cylinder already exists by virtue of application of the brake pedal, in TCS, the pressure must be supplied by the high pressure pump or, early on in the cycle, from a high pressure accumulator as the brake pedal is normally not active during acceleration.
In braking systems where the front and rear brake pressure is not proportioned, a low resistance flow path already exists between the master cylinder and the high pressure pump. However, in many vehicles, it is necessary to supply proportionately less pressure to the front wheel slave cylinders or rear wheel slave cylinders, generally the latter, and for this purpose it is necessary to include a brake fluid pressure proportioning valve in the master cylinder line when used with TCS. This would be an example of a vertically split system where the front and rear brakes are supplied brake fluid from a respective separate line, pressure chamber and reservoir of the master cylinder. The previously known proportioning valves present a flow restriction and this can momentarily deprive the pump of fluid particularly on systems without a high pressure accumulator.
Proportioning valves used in vehicular braking systems are designed such that both front and rear brake pressure increase in tandem up to the so-called "split-point". Above this pressure, the pressure associated with one set of brakes increases in step with the increase in master cylinder pressure, while the pressure associated with the second set of brakes increases at a diminished rate. This situation is reflected by FIG. 1a. In portion (A) of the plot in FIG. 1a, the pressure of both front and rear brakes (output pressure) increases uniformly, in step with the master cylinder pressure (input pressure). Above the split point (B), however, while the non-proportioned (e.g. front) brake pressure (C) continues to increase in direct proportion to the master cylinder pressure, the proportioned (e.g. rear) brake pressure (D) increases at a reduced rate.
A brake proportioning valve of the prior art is illustrated in FIG. 2. The valve is normally open. The plunger 1 is held against the off stop 2 of valve seat 5 by the force of the spring 3. The valve seat includes a plurality of radially extending ribs 4 at the off stop, namely the face opposite plunger shoulder 13. During brake apply, the flow of fluid from the master cylinder through port 7, below the split point, is between the radial ribs on the face of the valve seat and the shoulder 13 on the plunger. It then passes through the inner diameter or bore of the valve seat (as measured at enlarged plunger portion 11) to the outlet port 19.
The operation of the proportioning valve is described by the formula: P.sub.o A=Pi(A-B)+F, wherein
______________________________________ A = Cross sectional area of enlarged plunger portion 11 B = Cross sectional area of plunger at rear pin por- tion P.sub.i = Inlet pressure at port 7 P.sub.o = Outlet pressure at port 19 F = Force of spring 3 ______________________________________
When the hydraulic force acting on the plunger 1 exceeds the spring 3 force (P.sub.i B.gtoreq.F), the plunger moves away from the off stop until it contacts the valve seat 5. This occurs at the valve split point. This condition is depicted in FIG. 2. Now the plunger has closed against the seat, and fluid pressure acts against the plunger differential area (A-B). This produces an opening force on the plunger, assisting the spring, and delivers a portion of this increased pressure to the proportioned brakes through outlet port 19.
However, this increased pressure to the proportioned brakes creates an opposing force on the plunger, over area B, as the lip seal portion 21 of valve seat 5 is constructed with flow through clearance permitting reverse flow past the seal lip, thereby tending to reclose the plunger against the seat. These opposing forces tend to keep the plunger closely adjacent to the seat for the restricted flow of fluid from the inlet to the outlet. This flow is equal to the ratio of the annular area i.e. the plunger differential area (A-B) to the total area A within the effective valve seat sealing circumference, i.e. reducing Ratio=A-B/A.
If the master cylinder is maintained at a constant pressure (above split point), the plunger will remain closed, thereby keeping the proportioned brake pressure constant.
Devices similar to that shown in FIG. 2 are suitable for use in ABS, but are not optimal for TCS due to the limitations imposed on fluid flow. Moreover, the brake apply and brake release pressures curves are not similar. As shown in FIG. 1b, which represents the pressure in the brake apply and release modes, the output pressure during brake apply increases in step with the inlet pressure (A) up to the split point (B). Above the split point, the output pressure increases with input pressure, but at a different rate over the portion (C) of the plot, this rate determined by the differential areas A and B of the plunger as illustrated in FIG. 2. On brake release, however, the brake pressure is not instantaneously released, retracing the brake pressure apply curve, but rather, following an initial hysteresis due to O-ring seal friction (D), decreases slightly due to the limited volume compensation caused by the movement of the plunger away from the valve seat (E) toward the end stop, following which the pressure remains relatively constant (F) until the output pressure/input pressure differential is sufficient to force fluid around the periphery of the valve seat past the lip seal. At this point (G), the pressure again drops over area (H), and the plunger returns to its off stop against the valve seat when the spring force is greater than the hydraulic pressure (I). The result, which to a certain extent is dependent on ambient conditions, can be an extended period where the brakes may remain engaged despite master cylinder pressure having been released. The pressures during the brake apply and brake release cycles are not symmetrical, and designed proportioning is achieved only during the brake apply cycle.
It would be desirable to provide a proportioning valve which offers virtually unrestricted flow for use in TCS, and which provides a short shear path for fluid such that flow at low temperatures is maintained. It is further desirable to provide a proportioning valve which minimizes lag in brake release by providing a brake release pressure curve which closely mirrors the brake apply pressure curve, thus providing proportioning during both brake apply and brake release cycles.